1. Field of the Invention
The invention relates to a laser machining head for cutting work and a laser machining apparatus for controlling the laser machining head.
2. Description of the Prior Art
The cutting performance of a laser machining apparatus is affected by operating conditions such as the location of the focal point of the laser beam against the work surface, the assist gas pressure and the distance between the work surface and the leading edge of the laser beam nozzle. It also depends on conditions such as the kinds of material, surface condition, quality, composition, and thickness of the work. Regarding the laser machining apparatus, it is expected to work under a wide range of conditions and for a wide range of materials as described above and to obtain a stable machining quality. Especially, in case of cutting steel and so on, the metal which is melted by direct laser beam heating is not only blown off by an oxygen jet flow used as an assist gas, but is also sublimated or melted with a strong oxidized burning reaction, which improves cutting efficiency. Therefore, the cross sectional quality of the work surface very much depends upon oxidized burning speed on it.
Namely, the higher the velocity of oxidized burning, the less the roughness on the cross sectional surface. Since supplied oxygen is consumed continuously, self-burning (the phenomenon where strong self-burning happens explosively in the absent of a laser beam so that cross-sectional quality is damaged) due to excessive oxygen seldom happens. Also, since the cutting speed is increased, high speed cutting can be conducted. Therefore, it is very important to optimize the gas supplying condition and to increase oxidized burning speed. Based on the above situations, many kinds of improvement on gas supplying nozzle have been attained.
FIG. 31 is a longitudinal sectional view of a laser beam nozzle which is installed in the machining head of a conventional laser machining apparatus. The laser beam nozzle is disclosed, for example, in Japanese patent publication No. 61-60757. In the figure, the numeral 1 denotes a main assist gas nozzle, the numeral 2 denotes a sub assist gas nozzle, the numeral 3 denotes a surface of the material being cut, and the numeral 10 denotes an assist gas source.
The operation of laser beam machining apparatus is explained hereafter. The laser beam nozzle installed in a conventional laser beam machining apparatus is constructed as mentioned above. The nozzle has a multiple structure which comprises a gas flow path on the center axis and concentric multiple gas flow paths arranged around the center gas flow path. The oxygen supplied to the material being cut from the gas flow path on center axis, has comparatively high pressure and is discharged at a high speed. The oxygen is supplied mainly into the cutting groove and used partially for the oxidized burning reaction. The remaining oxygen is used to blow off and eliminate the melted and oxidized material. The oxygen supplied from the outside flow path flows at a comparatively low speed. The air, flowing from the outside, stabilizes the gas flow near the center axis and keeps the oxygen concentration at a high level. Moreover, the direction of jet flow from the outlet of the sub assist gas is arranged parallel to the outlet of the main assist gas and keeps the assist gas in a laminar flow. Since there are thick walls at the border of outlets of the main assist gas and the outlet of each sub assist gas, and also since the interference between the main assist gas and the sub assist gas is suppressed, main assist gas is kept in the laminar flow condition accordingly.
In the laser machining, it is necessary to supply continuously a required amount of assist gas (oxygen gas) into the narrow cutting groove whose width is less than 1.0 mm during cutting the work, although the oxidized burning reaction occurs due to the assist gas (oxygen gas) supplied to the cutting surface of the metal which is heated to a high temperature by the energy of the laser beam. Combustion products (gas or melted metal) are produced near the cutting surface due to the oxidized reaction. If the jet flow of the assist gas (oxygen gas) is in a laminar flow condition, the assist gas must reach the metal surface by dispersing through the ambient combustion product (gas composition). It is effective accordingly to make a jet flow of assist gas turbulent, to disturb the boundary layer in the combustion reaction area near the cutting surface and to replace the combustion product (gas composition) with fresh assist gas in order to increase the speed of combustion.
The laser beam nozzle installed in machining head of a conventional laser machining apparatus is arranged so that the jet direction of the sub assist gas outlet is parallel to that of the main assist gas outlet, and discharges the sub assist gas in a laminar flow, so that the interference with the main assist gas is greatly suppressed. Therefore, the main assist gas is kept in a laminar flow and stabilized. Although the purity of the assist gas (oxygen purity) is kept at a high level, since the gas flow at the center portion is stabilized, it is necessary to increase supply pressure of the main assist gas so as to effectively supply the assist gas (oxygen gas) into the narrow cutting groove. Since a boundary layer of law assist concentration (oxygen concentration) is formed on the oxidized reaction surface of the metal due to small velocity fluctuation (strength of turbulence) of the center portion of the nozzle flow, it is difficult for fresh assist gas (oxygen gas) to reach reaction surface directly. Therefore, there is a problem that the supplied assist gas (oxygen gas) does not contribute effectively to the oxidized reaction. Also, there is another problem that the self burning may occur when the oxygen which is not used effectively may remain at the narrow cutting groove due to some condition of the flow quantity. Since a thick wall exists at the boundary of the jet outlets of the main assist gas and each sub assist gas, the assist gas jet pressure or the jet velocity distribution from the front edge of the laser beam nozzle does not vary continuously. Accordingly, the jet flows of the main assist gas, each sub assist gas and the surrounding air are separated at the thick wall of their respective boundary. Therefore, there is a further another problem that the main assist gas purity (oxygen purity) decreases, since diffusion mixing improves between the main assist gas and the sub assist gas, and the isolation effect is decreased between the outermost sub assist gas and the surrounding air.